Automatic tuning system



WQR. KOCH AUTOMATIC TUNING SYSTEM Filed July 31, 1937 Feb. 13, 1940.

Patented Feb. 13, 1940 AUTOMATIC TUNING SYSTEM Wineld R. Koch, Merchantville, N. J., assignor to Radio Corporation o! America, a corpora- Y`tion of Delaware application Juiy 31, 1931, serial N0. 156,666

4 Claims.

The present invention relates to an automatic tuning system for radio receiving apparatus and the like, and has for its principal object to provide an improved and simplified automatic tuning system wherein the inductive and capacitive reactance of a series tuned signal circuit may be utilized to provide relatively variable controlling potentials for tuning motor and oscillator frequency control circuits, and the like.

It is also an object of the present invention to provide a system of the character above referred to, wherein a pair of rectifier devices are utilized in deriving the controlling potentials, and which may, at the same time, provide demodulation of an amplitude modulated signal or frequency modulated signal.

The control circuit of the invention is particularly adapted i'or use in a superheterodyne type of receiver in connection with the intermediate frequency amplifier, as a' combined second detector and automatic tuning control means.

In providing one type of automatic tuning control, as is well understood, the frequency of the local oscillator may be controlled to provide mixing at the selected intermediate frequency whenever the receiving system is approximately in tune with an incoming signal, or, in another type, the tuning element may be controlled automatically to exact resonance when approximate resonance with an incoming signal is obtained by manual or other tuning adjustment initiated by normal tuning operation. The present system may be applied 'to either type.

Tre invention will, however, be better understoociby reference to the accompanying drawing and the following description thereof, and the scope of the invention will be pointed out in the appended claims.

In the drawing,

Figure 1 is a schematic circuit diagram of a portion of a superheterodyne receiver provided with an automatic tuning control system embodying the invention,

Figure 2 is a graph showing curves illustrating a feature of the operation of the circuit of Fig. 1, and

Figure 3 is a schematic circuit diagram showing a modification of the control system of Fig. 1 and a further embodiment of the invention.

Referring to Fig. 1, an intermediate frequency amplifier output stage 5 is coupled through a suitable intermediate frequency output coupling transformer 6, to a series resonant circuit comprising a capacity C and an inductance L provided, respectively, by a variable tuning capacitor (Cl. lZ50-20) 1 and an inductive winding 8 connected in series across the secondary 9 of the intermediate frequency output transformer 8 in series with a limiting resistor I0.

This resistor affects the amount of current flowing through the circuit C and L, and by increasing the resistance, the sharpness of the control can be reduced to any desired value.

The mid-point of the series resonant circuit LC, between the tuning capacitor 1 and the inductive winding 8, as indicated at Il, is connected to ground or chassis indicated at i2 and to the midpoint i3 of an output resistor comprising two sections I4 and i5 for a pair of diode rectifier devices indicated at I6 and i1.

The resistor section i4 is connected in shunt with the capacity C through the diode rectifier device I6, and the section i5 is connected in shunt with the inductance L through the diode rectifier device I1, the rectifier devices being connected in series relation to each other, one to each end of the series tuned circuit LC, with the output resistor sections i4 and I5 connected between the rectifier devices in such a manner that the anode I8 of one rectifier device and the cathode i9 of the other rectifier device are connected to the series tuned circuit terminals, while the remaining cathode and anode 2| are connected through the resistor sections i4 and l5. Suitable intermediate'frequency by-pass capacitors, indicated at 22, are connected across the resistor sections.

The rectifier circuit is provided with output terminals and 26, across which is connected an output potentiometer resistor or impedance 21 having output taps indicated at 28 and 29. The tap 28 is adjustable along the potentiometer device 21 and in the present example isI connected through a coupling capacitor 30 to a suitable audio frequency amplifier stage, indicated by the amplifier tube 3i, from which audio frequency output signals are taken through the anode output circuit indicated at 32.

The tap 29 is a mid tap on the potentiometer 21, and is connected to any suitable frequency control element responsive to a variable direct current potential. In the present example, it is connected through a control lead 33 with the control grid 34 of an amplifier or control tube 35 having a. cathode 36 and an output anode 31. The yresistor network in connection with the rectiers is conductive to ground i2, and through this conductive connection and in series therewith a suitable source of biasing potential is provided for the control tube 35. In the present example.; this is provided by a suitable self-bias resistor 58. connected to ground or chassis 38, which is the negative anode current supply connection for the receiving system shown. The mid tap 25 is also connected to ground, indicated at 40, through a suitable by-pass capacitor".

.tive coupling between the lnductance 41 of the tuned circuit 45 and coil 52 in the anode circuit 58. The control is such that as a variable anode current is applied through the coil 52 by reason of a variable potential on the control grid 34 of the control tube, the oscillator circuit is variably loaded and the frequency varied within predetermined limits. f

Since the oscillator is the local oscillator of the receiver, it is connected to the rst detector (not shown).V through a potential supply lead indicated at 53. Likewise, the main tuning capacitor 48 is provided with connection, indicated by the dotted line 54, with the remainder of the gang tuning control for the receiver. As the oscillator and control therefor may be of any suitable type adapted to be embodied in a superheterodyne receiver as a local oscillator under control of a control tube such as the tube 35, further description is believed to be unnecessary.

In the control system of the present example, the voltages appearlngacross the series resonant circuit, reactances C and L are used to provide controlling potentials for the control tube 35 in the following manner.

At resonance for the desired intermediate frequency, the voltage appearing across the inductance L, provided by the winding 8, is made equal to that across the capacity C provided by the variable capacitor 1 by proper relation of the inductive and capacitive reactances. At frequencies below resonance, the voltage across C will be larger than that across L, and at frequencies above resonance, the voltage across L will be larger than that across C.

With constant current through the series resonant circuit, the voltage drop will be proportional to the reactances, represented by the graphs 60 and 8| of Fig. 2 being the inductive reactance and capacitive reactance, respectively, plotted across frequency. The desired operation is such that at a frequency F, such as the predetermined intermediate frequency, the inductive and capacitive reactances are substantially equal, as indicatedl by the points 63 and 64, respectively, corresponding to the frequency F.

The diode rectifier devices are connected so that the voltageacross Kthe capacity C tends to make the high potential or ungrounded end of the output resistor I4, and the output terminal '2 5, positive with respect to ground. The voltage across the lnductance L tends to make the ungrounded or high potential end of the resistor section I5, and the output terminal 26, negative with respect to ground. l

The control circuit 33 is connected with the mid tap output terminal 29, and the control grid 34 of the control tube 35 receives a positive grid voltage for applied signal frequencies less than 2,19o,s1a

resonance, tending to increase the plate current and load the oscillator tuned circuit 45 in one direction, and a negative grid voltage for applied signal frequencies above resonance, tending to decrease the anode current of the controltube Il and decreasing the load on the oscillator tuned circuit 45.

IIl'he change in load on the oscillator is such that in conjunction with a received signal it tends to restore the intermediate frequency to normal, whether the receiving circuit be tuned slightly above or slightly below resonance to the received signal, thereby effecting automatic tuning of the system.

By adjusting the tuning capacitor 1 to provide a reactance indicated by the dash line 85 in Fig. 2, the inductive and capacitive reactances may be made equal at a differing frequency F1, as indicated by the points 88 and 81. -In this manner, by adjusting one of the reactance elements o f the series resonant circuit from which the diode rectiflers are supplied, the frequency response of the tuning control system may be adjusted within limits determined by the frequency response range of the intermediate frequency amplifier or other tuned signal channel to which it may be connected. This form of the invention has the advantage that no center tapped coils are required, no radio choke coils are involved, and operation over a relatively wide range may be realized.

Likewise, the control of frequency or tuning may be effected by suitable relay or motor means under control of suitable tubes, for example, as in a circuit shown in- Fig. 3, to which attention is now directed, and in which a pair of thermionic tubes 10 and 1| are connected to control the ow of alternating current from mains indicated at 12 through an output transformer 13, to one winding 14 of an alternating current motor 15, another winding 16 of which is excited from the mains through a capacitor 11. The motor is of the two-phase type as indicated, and is connected with any suitable variable tuning means for a receiving system through the shaft indicated at 18.

The tube 10 receives a controlling potential through a control lead which may be connected to the point A or terminal 29 of the control system of Fig. l insubstantially the same manner as the control lead 33 for the tube 35, previously described. In the present example, however, the control grid 8| of the tube 10 is connected directly with the control lead 80 to receive a biasing and control potential therefrom, while the control lgrid 82 of tube 1| is connected to the chassis or negative terminal of the anode voltage supply source 81.

When the voltage applied between the grid 8| of tube 1.0 and ground is zero, equal biases will exist for tubes 10 and 1|. Equal anode currents will flow, in opposite directions, through each half of the primary or tube side of transformer 13, and no voltage will be induced in the secondary. The motor will not turn. If a nega.- tive voltage is applied to lead 88 by the control circuit, tube 10 will be biased more than tube 1|, and the anode current through the primary of transformer 13, connecting with 1|, will be greater than that through the other half. A voltage will be induced in the secondary and applied to motor Winding 14, of proper phase to turn the motor in one direction. A positive voltage applied to lead 80 will result in a voltage applied to 14, but of opposite phase, turning motor in the opposite direction. By proper connections, the receiver can be made self-tuning. This modification of the invention has the advantage that both the radio frequency stages and oscillator are automatically tuned.

I claim as my invention:

1. In a radio signalling system including a rst detector', a local oscillator and an intermediate frequency stage, the. combination of a frequency discriminating control circuit comprising series resonant tuning elements provided with end and intermediate terminals and connected in parallel with the output of said intermediate frequency stage, a resistance network provided with end and intermediate terminals, a pair of rectifying elements reversely connected between the end terminals of said circuit and said network, means interconnecting said intermediate terminals, means for automatically tuning said oscillator in response to change in the potential cf said network, and means for deriving an audio frequency output from said network.

2. In a radio signal receiving system, the ccmbination of an intermediate frequency amplifier having an output transformer provided with a secondary winding, a tuning inductance and capacitor providing a series resonant circuit connected across said secondary winding, a current regulating resistor connected between said secondary winding and said series resonant circuit, at least one o f said series resonant circuit elements being adjustable whereby the signal potential across said elements is substantially equal at a predetermined intermediate frequency, a pair of diode rectier devices connected in opposing relation to each other with the terminals of said series resonant circuit and having series connected output resistors, a grounded tap connection between said resistors connected with said series resonant circuit between the tuning elements thereof, an output resistor having a mld-tap and having terminals connected with the end terminals of said series connected diode output resistors, a control tube having' a grid -electrode connected with the mid-tap of said output resistor and having a cathode connected to ground, said control tube having an anode secondary winding, a tuning inductance and capacitor providing a series resonant circuit conments being adjustable whereby the signal po eul tial across said elements is substantially equal at a predetermined intermediate frequency, a pair of diode rectifier devices connected in opposing relation to each other with the terminals of said series resonant circuit and having series connected output resistors, a grounded tap connection between said resistors connected with said series resonant circuit between the tuning elements thereof, an output resistor having a midtap and having terminals connected with the end terminals of said series connected diode output resistors, a control tube having a grid electrode connected with the mid-tap of said output resistor and having a cathode connected to ground, said control tube having an anode outputy circuit, and means including a variable tap connection on said output resistor for deriving an audio frequency therefrom.

4. In a superheterodyne receiver, the combination with an oscillator and an intermediate frequency amplifier, of an output circuit for said intermediate frequency, a. tuning capacitor and a tuning inductance providing a series resonant circuit in shunt with said output circuit, one of said circuit elements being adjustable to provide substantially equal signal voltages across said elements at resonance to a predetermined intermediate frequency, a pair of diode rectier devices connected in series between the terminals of said series resonant circuit, a pair of diode output resistors connected between said rectiers, means providing a connection between the junction of said output resistors and the junction of said series resonant circuit elements, said last named connection being grounded, a potentiometer device connected between the terminals of said series connected output resistors, an intermediate tap on said potentiometer providing an output terminal for said rectifiers, a control tube connected with said output terminal to receive a direct current controllingpotential therefrom, and means coupling said control tube with said oscillator whereby the frequency of said oscillator is varied in response to variations in said controlling potential.

WINFIELD R. KOCH. 

